The present invention relates to apparatus and methods for determining the position and orientation of an object by means of magnetic fields, and includes apparatus and methods for monitoring the position of a probe within the body of a medical patient by means of magnetic fields.
Various methods have been utilized for determining the position and orientation of an object in space. For example, it is often necessary to determine the position and orientation of a medical instrument within the body of a patient as, for example, to monitor the position and orientation of a catheter, endoscope or other probe. It is desirable to present data showing the object superposed on a picture of the patient such as on an image showing internal organs. One simple method of accomplishing this is to take a direct X-ray image, such as a fluoroscopic image, showing both the patient and the inserted probe. This method has the disadvantage that the entire imaging procedure to acquire an image of the patient must be repeated whenever the probe is moved. X-ray imaging such as fluoroscopy can be performed during some surgical procedures, but inherently expose the patient to unwanted ionizing radiation. Some types of imaging procedures, such as MRI imaging cannot be repeated during surgery or other treatment procedures.
As illustrated for example in GB patent application 2,094,590 and in U.S. Pat. No. 5,186,174, the probe may be mechanically constrained or linked to an arm or frame so that the position of the probe is constrained with respect to a fixed frame of reference and known with respect to that frame of reference. An image of the probe may be superposed on an image of the patient, using the positional data derived from the fixed frame of reference. However, these systems make the probe inflexible and hence impose severe disadvantages in the medical procedure itself. For example, one such system requires that a probe be advanced in a straight line to the area of interest. Systems of this nature are more suitable for positioning a probe on the outside of the body than inside the body.
Bryer et al. U.S. Pat. No. 4,697,595 and Bryer et al., Ultrasonically Marked Catheter--a Method for Positive Echographic Catheter Position Identification, Medical and Biological Engineering and Computer, vol. 22, No. 3 (1984), pp. 268-271, both disclose intracardiac catheters equipped with ultrasonic detectors. The position of the catheter is deduced from time of flight measurements from an ultrasonic transducer on the exterior of the patient to the catheter, and the deduced position is superimposed on an ultrasonically generated image.
Van Steenwyk et al. U.S. Pat. No. 4,173,228; Pfeiler et al. U.S. Pat. No. 5,042,486 and Dumoulin et al. U.S. Pat. No. 5,211,165 all disclose arrangements wherein electromagnetic signals are propagated between one antenna on the tip of a medical catheter inserted in the body and several antennas outside of the body. The position and orientation of the catheter tip are assertedly determined from the signals transmitted between these antennas. That is, the relative position and orientation is deduced from the properties of the signal propagation path between these antennas as, for example, from the degree of signal attenuation in transit from one antenna to the others. The Van Steenwyck patent notes the possibility of using a magnetic field and a Hall effect transducer sensor, but offers no details as to how this might. be accomplished in a practical device. Dumoulin suggests that the radio frequency derived position of a catheter tip can be superposed on an image acquired by an imaging system.
Numerous systems have been proposed for locating articles outside of the human body by use of magnetic fields. Thus Blood, U.S. Pat. Nos. 4,945,305; 4,849,692 and 4,613,866 all disclose systems for determining the position and orientation of objects in three-dimensional space using magnetic coils on the object to be located and stationary coils in a fixed frame of reference. Other systems of this type include Voisin U.S. Pat. Nos. 5,172,056 and 5,168,222; Constant U.S. Pat. No. 4,396,885; Cantaloube U.S. Pat. No. 5,109,194; Weed et al. U.S. Pat. No. 4,317,078; Hansen U.S. Pat. No. 4,642,786 and Morgenstern U.S. Pat. No. 5,047,715. These systems typically employ a magnetic field transmitter incorporating several coils wound on orthogonal axes about an iron core, and a similar structure used as a receiver. The coils of the transmitter are actuated in sequence and/or at different frequencies, and the signals detected by the coils of the receiver are analyzed to determine the position and orientation of the transmitter relative to the receiver. Among the uses for such systems are three-dimensional data entry devices for computers and systems for detecting the position and orientation of a helmet.
Additionally, as disclosed in Remel, An Inexpensive Eye Movement Monitor Using the Scleral Search Coil Technique, IEEE Transactions On Biomedical Engineering, vol. BME-34, #4 April 1984, pp 388-390, researchers attempting to follow the rotation of the eyeball have mounted small loop-like sensing coils on the surface of the eyeball, as by suturing or by incorporating the coil in a contact lens. The subject having such a coil is placed between pairs of orthogonally oriented Helmholtz coils which are energized with high-frequency alternating currents having two different frequencies. The voltage induced in the coil on the eye will include components at both frequencies, and the relative magnitudes of these components will depend upon the orientation of the eye.
Despite all of these efforts in the art, there still has been a need heretofore, for improved apparatus and methods for determining the position and orientation of an object in space, and, particularly, for improved apparatus and methods for determining the position and orientation of a probe within the body of a living subject.